ADXRS642 Analog Devices, ADXRS642 Datasheet - Page 6

• Manufacturer Part Number: ADXRS642
• Description: Vibration Rejecting Yaw Rate Gyro
• Manufacturer: Analog Devices

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ADXRS642
THEORY OF OPERATION
The ADXRS642 operates on the principle of a resonator gyro.
Figure 4 shows a simplified version of one of four polysilicon
sensing structures. Each sensing structure contains a dither
frame that is electrostatically driven to resonance. This
produces the necessary velocity element to produce a Coriolis
force when experiencing angular rate. The ADXRS642 is
designed to sense a Z-axis (yaw) angular rate.
When the sensing structure is exposed to angular rate, the
resulting Coriolis force couples into an outer sense frame,
which contains movable fingers that are placed between fixed
pickoff fingers. This forms a capacitive pickoff structure that
senses Coriolis motion. The resulting signal is fed to a series of
gain and demodulation stages that produce the electrical rate
signal output. The quad sensor design rejects linear and angular
acceleration, including external g-forces and vibration. This is
achieved by mechanically coupling the four sensing structures
such that external g-forces appear as common-mode signals
that can be removed by the fully differential architecture
implemented in the ADXRS642.
The electrostatic resonator requires 18 to 20V for operation.
Because only 5V are typically available in most applications,
a charge pump is included on chip. If an external 18 to 20V
supply is available, the two capacitors on CP1 to CP4 can be
omitted, and this supply can be connected to CP5 (Pin 6D,
Pin 7D). CP5 should not be grounded when power is applied to
the ADXRS642. No damage occurs, but under certain conditions,
the charge pump may fail to start up after the ground is removed
without first removing power from the ADXRS642.
Z
X
Figure 4. Simplified Gyro Sensing Structure – One Corner
Y
Rev. Pr. A | Page 6 of 10
SETTING BANDWIDTH
External Capacitor C
chip R
of the ADXRS642 rate response. The −3 dB frequency set by
R
and can be well controlled because R
during manufacturing to be 180 kΩ ± 1%. Any external resistor
applied between the RATEOUT pin (1B, 2A) and SUMJ pin
(1C, 2C) results in
In general, an additional filter (in either hardware or software)
is added to attenuate high frequency noise arising from demodu-
lation spikes at the 18 kHz resonant frequency of the gyro. An
R/C output filter consisting of a 3.3k series resistor and 22nF
shunt capacitor (2.2kHz pole) is recommended.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyros to improve
their overall accuracy. The ADXRS642 has a temperature propor-
tional voltage output that provides input to such a calibration
method. The temperature sensor structure is shown in Figure .
The temperature output is characteristically nonlinear, and any
load resistance connected to the TEMP output results in decreasing
the TEMP output and its temperature coefficient. Therefore,
buffering the output is recommended.
The voltage at TEMP (3F, 3G) is nominally 2.5 V at 25°C, and
V
Although the TEMP output is highly repeatable, it has only
modest absolute accuracy.
SUPPLY RATIOMETRICITY
The AD642’s RATEOUT, ST1, ST2, and TEMP signals are
ratiometric to the V
sensitivity, and temperature outputs are proportional to V
So, it is most easily used with a supply-ratiometric ADC which
results in self cancellation of errors due to minor supply
variations. There is some small, usually negligible, error due to
non-ratiometric behavior. Note that, in order to guarantee full
rate range, V
OUT
RATIO
and C
R
f
OUT
OUT
= 5 V. The temperature coefficient is ~9 mV/°C at 25°C.
OUT
resistor to create a low-pass filter to limit the bandwidth


OUT
/ 1

RATIO
180
is

2
Figure 5. Temperature Sensor Structure

kΩ
should not be greater than AV
V
π
RATIO
RATIO
Preliminary Technical Data

OUT

R
R
FIXED
R
OUT
EXT
is used in combination with the on-
voltage, i.e., the null voltage, rate
 

/
180
R
C
TEMP
OUT
kΩ

OUT

V
R
TEMP
has been trimmed
EXT

CC
RATIO
.